test(package-deps): snapshot serialized subsystem dependencies

test/blackbox-tests/test-cases/package-materialization/subsystem-dependencies.t

@@ -0,0 +1,63 @@
+Installed dune-package metadata can refer to libraries through public subsystem
+fields that are not ordinary `requires`. A scoped package dependency currently
+omits those referenced libraries.
+
+  $ make_dune_project 3.24
+  $ cat >>dune-project <<EOF
+  > (package (name subsystem-root))
+  > (package (name subsystem-runner))
+  > EOF
+
+  $ mkdir backend runner consumer
+  $ cat >backend/dune <<'EOF'
+  > (library
+  >  (name backend)
+  >  (public_name subsystem-root.backend)
+  >  (modules ())
+  >  (inline_tests.backend
+  >   (runner_libraries subsystem-runner)
+  >   (generate_runner (echo "let () = print_endline Runner.message"))))
+  > EOF
+
+  $ cat >runner/dune <<'EOF'
+  > (library
+  >  (name runner)
+  >  (public_name subsystem-runner))
+  > EOF
+  $ cat >runner/runner.ml <<'EOF'
+  > let message = "subsystem dependency"
+  > EOF
+
+  $ cat >consumer/dune-project <<'EOF'
+  > (lang dune 3.24)
+  > EOF
+  $ cat >consumer/dune <<'EOF'
+  > (library
+  >  (name tested)
+  >  (inline_tests (backend subsystem-root.backend)))
+  > EOF
+  $ echo 'let value = ()' >consumer/tested.ml
+
+  $ cat >dune <<'EOF'
+  > (rule
+  >  (target result)
+  >  (deps
+  >   (package subsystem-root)
+  >   (source_tree consumer))
+  >  (action
+  >   (with-stdout-to %{target}
+  >    (chdir consumer (run %{bin:dune} runtest)))))
+  > EOF
+
+  $ dune build result 2>err
+  [1]
+  $ censor <err
+  File "$PWD/_build/install/default/.packages/$DIGEST/lib/subsystem-root/dune-package", line 35, characters 20-36:
+  35 |   (runner_libraries subsystem-runner)
+                           ^^^^^^^^^^^^^^^^
+  Error: Library "subsystem-runner" not found.
+  -> required by
+     _build/default/.tested.inline-tests/.t.eobjs/native/dune__exe__Main.cmx
+  -> required by _build/default/.tested.inline-tests/inline-test-runner.exe
+  -> required by alias runtest-tested in dune:3
+  -> required by alias runtest in dune:1